Voltage surge diverter



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United States Patent O U.S. Cl. 317-68 16 Claims ABSTRACT F THEDISCLOSURE A current limiting gap lightning arrester with extra valveresistance for raising the discharge voltage at the beginning of thedischarge and time delayed acting means for effectively shunting out theextra valve resistance to lower the discharge voltage at an appropriatetime after the start of the discharge. The time delayed acting means maybe a magnetic blow out series gap with auxiliary electrodes shunting theextra valve resistance which electrodes are bridged after a time delayby the magnetically blown out arc. The current limiting gaps may bedisposed in parallel columns for flip-flop operation and fast actingmeans may be employed for shunting out the extra series valve resistancein response to impulse discharge current of large value.

This invention relates to voltage surge diverters and more particularlyto improvements in valve type lightning arresters with current limitinggaps or gap units, especially such arresters for protecting high voltagedirect current power systems.

A valve type lightning arrester comprises one or more valve resistors inseries with one or more spark gaps. A valve resistor has aninstantaneous negative resistancecurrent characteristic. A currentlimiting gap has a noninstantaneous or delayed rising voltagecharacteristic after sparkover. As arrester voltage during discharge,i.e., after sparkover and before seal off or interruption of powerfollow current, is the sum of the valve and gap voltages it is customaryto use less valve resistance with current limiting gaps than withordinary gaps in order to limit the arrester voltage during discharge.Current limiting gaps are particularly useful in arresters for directcurrent circuits because of their ability to seal 01T against the thenon-cyclic circuit voltage.

However, in certain applications of valve type current limiting gaparresters the low discharge voltage immediately after sparkover isundesirable; for example, when such arresters are used to protectelectronic or ionic valves in the rectifying and inverting converters atthe ends of a high voltage direct current power transmission line. Insuch converters it is customary to bridge connect the electronic valvesand series-parallel connect the bridges on their direct current sides.The paralleling is done by bus conductors and the series connectionpermits operation of the line at a voltage which is a multiple of theindividual valve voltages. The arresters are connected between thebusses and ground. In such an arrangement, if an arrester connected toan intermediate bus is sparked over by a voltage surge its low dischargevoltage pulls the bus voltage way down but the line voltage cannotchange that fast due to stray capacitance in the circuit. Consequentlythe electronic valves between the line and the lowered voltage bus aresubjected to excessive voltage causing them to arc back and fail.

Merely increasing the valve resistance of the arrester is no solution ofthe problem because that will increase the arrester voltage during thedischarge period thus impairing the protective characteristics of thelightning arrester.

In accordance with this invention, novel time delayed acting means isprovided for effectively shunting out extra valve resistance. In thismanner, the arrester is prevented from having too low a dischargevoltage before the current limiting gap has time to build up substantialvoltage and afterthat time the arrester is prevented from having toohigh a discharge voltage by the shunting out of the extra valveresistance.

An object of the invention is to provide a new and irnproved voltagesurge diverter.

Another object of the invention is to provide a new and improved valvetype lightning arrester with current limiting gaps.

A further object of the invention is to control the discharge Voltage ofa valve type arrester with current limiting gaps at both ends of itsconduction period.

An additional object of the invention is to raise the discharge voltageimmediately after sparkover and lower the discharge Voltage before sealoff or clearing of a valve type lightning arrester having a currentlimiting series gap or gaps.

A still further object of the invention is to provide novel slowshunting means for extra valve resistance in a valvetype lightningarrester.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings and its scope will bepointed out in the appended claims.

In the drawings:

FIG. 1 is a schematic circuit diagram of an embodiment of the invention,

FIG. 2 is an exploded detailed view of a portion of the time delayedacting means,

FIG. 3 is a modification in which the current limiting gaps are inparallel columns for producing a ip-flop mode of operation with some ofthe valve resistance in the columns and with the rest of the valveresistance and the extra resistance in the common circuit of thearrester,

FIG. 4 is a diagram of a modified form of time delayed acting means, and

FIG. 5 is a still further modification of the time delayed acting means.

Referring now to the drawings and more particularly to FIG. 1, there isshown therein in schematic diagrammatic form a valve type lightningarrester 1 comprising a current limiting gap unit or units 2 in serieswith a normal valve resistor 3. In addition, there is an extra seriesvalve resistor 4 and a special slow acting shunting means 5 for theextra resistor 4. The terminals of the arrester are indicated at 6 and 7either one of which may be connected solidly to ground and the otherconnected to a power circuit element to be protected againstovervoltage.

The slow shunting means 5 is shown as comprising a horn gap 8 havingcharacteristic curved electrodes with an intermediate sparkover spacingand diverging are runners. In addition, the horn gap 8 is provided witha pair of auxiliary electrodes 9 insulated from each other and from themain electrodes of the horn gap 8 but in the path of the magneticallyblown out arc between those main electrodes. The auxiliary electrodesare connected in shunt circuit relation with both the extra valveresistor 4 and the horn gap electrodes 8. In addition, there is provideda resistor 10 of typically a few thousand ohms for carrying the gradingcurrent of the arrester, the voltage grading network of which has notbeen shown as it is not a part of the present invention.

A suitable structural form of the slow shunting gap is shown in FIG. 2in which the electrodes of the horn gap 8 are mounted on a ceramic disc11 which is provided with an auxiliary insulated electrode 9 and anotherauxiliary nsulated electrode 9 mounted on a face of another ceramic disc12. These two faces of the discs 11 and 12 when the discs are placedtogether form an arc chamber for the arc of the horn gap 8. The resistoris shown connected across the gap 8.

The current limiting gap unit 2 may be of any suitable type such, forexample, as is fully described in Pat. 3,320,482 granted May 16, 1967 ona joint application of Eugene C. Sakshaug and Earl W. Stetson filed June2, 1964 and assigned to the present assignee.

The operation of the invention shown in FIG.1 is as follows. When asufficiently high voltage surge is impressed across the terminals of thearrester all of its gaps spark over and discharge current flows throughthe arrester. Because of the extra valve resistor 4, the arrester isprevented from having too low a discharge voltage. As current continuesto fiow through the horn gap 8, the curvature or loop in the currentpath feeding its arc produces an electromagnetic force on the arc,moving or blowing it out along the arc runners to the right as viewed inthe drawing. Successive positions of the blown out arc at the ends ofthe arc runners are indicated by the wavy lines at 13 and 14. Finallythe blo'wn out arc will bridge the electrodes 9, whose spacing issomewhat greater than the sparkover distance of the electrodes of thehorn gap 8. When this happens, the arc 15 forms between the auxiliaryelectrodes 9 and the horn gap arc is extinguished. The arc 15 shunts oreffectively short circuits the extra valve resistor 4 as well as thehorn gap 8. Consequently, toward the end of the arresters dischargeperiod or at about the time that the discharge current of a switchingsurge begins to subside, the extra valve resistor is removed from thearrester circuit because at that time the current limiting gap 2 hasbuilt up suicient voltage for interrupting follow current and sealingoff the arrester.

In the modification shown in FIG. 3 there is shown a more complicatedarrester 16 having a so-called flip-Hop portion 17 and a common portion18. The flip-flop portion consists of two parallel columns eachcomprising at least one current limiting gap 2 and at least one valveresistor 19. The common portion 18 which, of course, is in series withthe flip-Hop portion 17 comprises a non-current limiting gap 20, acommon valve resistor 21 and an extra valve resistor 4 corresponding tothe extra valve 4 of FIG. l. The valve resistor 4 is arranged to beshunted by a slow shunting gap unit 5 similar to the element 5 ofFIG. 1. In addition, all or part of the common resistance is shunted bya current switching gap unit 22. The details of the non-current limitinggap and the current switching gap 22 are not shown as they form no partof the present invention. Suitable examples of such gaps are disclosedin the above identified Sakshaug et al. patent.

The concept of parallel columns of current limiting gaps having aflip-flop mode of operation is the subject matter of another inventionand is per se disclaimed herein. Briefly, the operation is such that thecolumns do not conduct simultaneously but alternately because upon theoccurrence of an overvoltage one current limiting gap will naturallyspark over before the other. However, as its voltage builds up afterbreakdown or sparkover, the other gap in the parallel column will besubjected to the full voltage of its parallel column, i.e., the built upvoltage of the current limiting gap and the voltage drop in the valveresistor in series with the latter so that when that voltage exceeds thesparkover voltage of the non-sparked over gap in the parallel column,the latter will sparkover thus-clearing the first column and initiatingthe second half of a cycle of flip-flop operaton.

The operation of the extra valve resistor 4' and the slow shunting gapunit 5 is essentially the same in FIG. 3 as in FIG. 1 in that the extravalve resistor 4 prevents the discharge voltage of the arresterparticularly in its initial stages, from being too low but after acertain length of time which is long in comparison with the duration ofan ordinary impulse but which is short in comparison with the durationof an ordinariy switching surge, the unit 5 will shunt out the extravalve resistance.

In order to prevent the arrester from having too high a dischargevoltage during the flow of large impulse discharge currents which are ofshort duration, the fast shunting current switching gap 2.2 willsparkover lwhenever the voltage drop across the resistors 4' and 21exceeds a predetermined value, but this gap like a current limiting gapwill in a time 'which is short in comparison with the duration of theswitching surge build up substantial voltage thus switching the lowermagnitude but longer duration switching surge discharge current into theresistors 4' and 21.

FIG. 4 is a modified form of slow shunting gap unit 5 in which the extravalve resistor is in the form of three separate resistors 4a, 4b, and4c, 4a and 4c being connected on one side of the horn gap 8 and 4b beingconnected on the other side of the horn gap 8, the auxiliary electrodes9 shunting the entire assembly 5 and there being a magnetic coil 23 foraugmenting the magnetic blowout action of the gap 8, the coil 23 beingconnected across the horn gap 8 and the resistor 4c. The purpose of thelatter is to augment the action of the coil 23 by increasing its voltageand hence its flux. It will be understood that the polarity of the coil23 is such that its magnetic flux drives the arcs 13 and 14 of the horngap 8 out along the horns, i.e. in the same direction that they arepropelled by the inherent action of the horn gap itself.

In the modification 5" shown in FIG. 5 only one extra valve resistor 4is employed, the extra valve resistors 4b and 4c being omitted and thecoil 23 being connected directly across the horn gap 8. 4In this figurethe horn gap action is relied on entirely for starting the blow outaction of the arc whereas in FIG. 4 the extra resistor 4c tends toovercome the iductive time lag effect of the coil 23 and cause fastercoil produced starting of the arc motion.

In both FIGS. 4 and 5, however, the arc motion is more positive becauseof the presence of the coil than in FIGS. 1 and 2 which rely entirely onthe horn gap effect.

It will, of course, be obvious that either the modification 5 or themodification 5" can be .substituted for the slow shunting gap unit 5 inFIGS. 1 and 3.

While there have been shown and described particular embodiments of theinvention, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention, andtherefore, it is intended by the appended claims to cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In a Valve type lightning arrester with at least one current limitingseries gap, and at least one valve resistance, an extra valveresistance, and means for effectively shunting out the extra valveresistance after a time delay following sparkover ofthe current limitinggap and before the current limiting gap clears and seals ofl".

2. An arrester as defined in claim 1 in which said means is a magneticblow out series gap with auxiliary electrodes insulated from each otherand from the main electrodes but in the path of the blown out arcbetween the main electrodes shunting said extra valve resistance andmagnetic blow out series gap.

3. An arrester as defined in claim 2 in which said magnetic blow outseries gap is a horn gap.

4. An arrester as defined in claim 2 in which the magnetic blow outseries gap is shunted by conductive means for passing grading current.

5. An arrester as defined in claim 4 in which said conductive means is aresistor. p

6. An arrester as defined in claim 4 in which said conductive means is amagnetic coil for also blowing out the arc.

7. An arrester as defined in claim 2 in which said extra valveresistance is in the form of two resistors respectively connected onopposite sides of said magnetic blow out series gap.

8. An arrester as defined in claim 6 in which said coil shunts both saidmagnetic blow out gap and at least a part of said extra valveresistance.

9. An arrester as defined in claim 6 in which said extra valveresistance is in the form of three resistors, two of which are connectedon one side of said magnetic blow out gap and the other of which isconnected on the other side of said magnetic blow out gap, said coilshunting both said magnetic blow out gap and the nearest of the twoextra valve resistors connected on one side of said magnetic blow outgap.

10. An arrester as delined in claim 1 with a current switching gap alsoin shunt circuit relation to said extra resistance.

11. In a valve type lightning arrester, a pair of parallel columns eachcomprising at lea-st one current limiting gap and one valve resistor,said columns having a flip-Hop mode of operation during arresterdischarge, common series valve resistance and means for effectivelyshunting out at least a portion of the common valve resistance after apredetermined time delay following initiation of discharge through thearrester but before the arrester seals off and terminates the discharge.

12. An arrester as defined in claim 11 in which said common series valveresistance is in the form of a normal resistor and an extra resistor andsaid means shunts out only the extra resistor.

13. An arrester as defined in claim 11 having a common series noncurrentlimiting gap.

14. An arrester as defined in claim 12 having a currentl ReferencesCited UNITED STATES PATENTS 1,902,510 3/1933 McEachron et al. 317-703,019,367 l/l962 Kalb 315-36 3,320,482 5/1967 Sakshaug et al 317-69 X3,348,100 10/1967 Kresge 317-68 X JAMES D. TRAMMELL, Primary ExaminerU.S. Cl. X.R.

